This invention relates generally to magnetic resonance imaging (MRI) and spectroscopy, and more particularly the invention relates to simultaneous spatial and spectral selectivity in MRI using a single excitation pulse.
Nuclear magnetic resonance (NMR) imaging, also called magnetic resonance imaging (MRI), is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
A descriptive series of papers on NMR appeared in the June 1980 issue of the IEEE Transactions on Nuclear Science, Vol. NS-27, pp. 1220-1255. The basic concepts are described in the lead article, "Introduction to the Principles of NMR," by W. V. House, pp. 1220-1226, which employ computed tomography reconstruction concepts for reconstructing cross-sectional images. A number of two- and three-dimensional imaging methods are described. Medical applications of NMR are discussed by Pykett in "NMR Imaging in Medicine," Scientific American, May 1982, pp. 78-88, and by Mansfield and Morris, NMR Imaging in Biomedicine, Academic Press, 1982.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90.degree. or 180.degree.. After excitation the nuclei gradually return to alignment with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
The excitation frequency, and the FID frequency, is defined by the Larmor relationship which states that the angular frequency, .omega..sub.o, of the precession of the nuclei is the product of the magnetic field, B.sub.o, and the so-called magnetogyric ratio, .gamma., a fundamental physical constant for each nuclear species: EQU .omega..sub.o =B.sub.o .multidot..gamma.
Accordingly, by superimposing a linear gradient field, B.sub.z =z.multidot.G.sub.z, on the static uniform field, B.sub.o, which defines the Z axis, for example, nuclei in a selected X-Y plane can be excited by proper choice of the frequency spectrum of the transverse excitation field applied along the X or Y axis. Similarly, a gradient field can be applied in the X-Y plane during detection of the FID signals to spatially localize the FID signals in the plane. The angle of nuclei spin flip in response to an RF pulse excitation is proportional to the integral of the pulse over time.
For a number of imaging applications one would like to selectively examine a particular spatial slice and a particular spectral component of the object at the same time. The most important example of this is two-dimensional water/fat imaging. Water/fat imaging may be desirable as an end in itself, for example as a tool for examining atherosclerotic plaque. It may also be desirable to select for water or fat in order to avoid image artifacts, such as those encountered in rapid imaging sequences. Rapid imaging sequences based on steady-state free precession suffer from artifacts at water/fat boundaries. Rapid k-space scanning sequences can suffer intolerable shifts or blurring of either water or fat.
Many techniques for forming water/fat images using spectrally-selective excitation sequences have been studied. Most of these techniques combine a spatially-selective pulse with an additional spectrally-selective pulse; however, multi-slice acquisition is impossible with these techniques. One recent technique uses two offset spatially-selective pulses. However, for many applications a single pulse that is simultaneously spectrally selective and spatially selective would be preferable to a combination of pulses.
A k-space interpretation of small-tip excitation is given by Pauly, Nishimura, and Macovski in "A k-space Analysis of Small-Tip-Angle Excitation," Journal of Magnetic Resonance 81, 43-56 (1989). The present invention uses this k-space interpretation of small-tip-angle excitation to provide a single pulse that is simultaneously spatially and spectrally selective.